1. Field of the Invention
This invention relates to a method for the production of a fiber of such a material as metal oxide and an optoelectronic modulator. More particularly, this invention relates to a method for the production of a fiber in accordance with the sol-gel technique and an optoelectronic modulator using the fiber so obtained.
2. Related Art
Researches have been heretofore under way concerning the applications of metal oxides for functional elements by virtue of various characteristic properties owned by the metal oxides. For example, (Pb, La) (Zr, Ti)O.sub.3 (hereinafter referred to as "VPLZ") is known to possess a crystalline structure of the perovskite type and manifest a prominent secondary optoelectronic effect (Kerr effect).
If such metal oxide is enabled to form a fiber, the range of applications of the metal oxide to the products utilizing the characteristic properties mentioned above will be widened and the development of new devices using the fiber will be realized.
As a means for manufacturing a metal oxide into a fiber, a method of production by means of a solid-phase reaction which comprises mixing the oxides or carbonates of prescribed component elements, subjecting the resultant mixture to a pyrogenic reaction in the air, and sintering the produced powdery raw material has been known. The solid-phase reaction, however, is such that the component elements are not intermingled to the molecular level homogeneously and the resultant mixture, when sintered, tend to produce an impure phase or a heterogeneous portion in the bulk because the powder raw material produces a fiber having a diameter of the order of microns only with extreme difficulty and because the reaction suffers metal ions from the oxides or carbonates of the component elements to migrate past grain boundaries. Further, since this method utilizes a reaction at an elevated temperature, it consumes a large volume of energy and proves expensive from the viewpoint of industrial chemistry.
Further, the "sol-gel method" for obtaining a synthetic metal oxide fiber by hydrolyzing or polymerizing the raw material in the state of a sol, one form of the liquid state, into the fiber in the state of a gel has been also known. The metal oxide synthesized by the sol-gel method enjoys homogeneity to the molecular level and assumes a state rich in density and scanty of an impurity phase as compared with the bulk synthesized by the solid-phase reaction. Further, since the reaction occurs in a liquid state, it consumes energy only in a small amount and proves advantageous in terms of cost as compared with the solid-phase reaction.
The sol-gel method has been known from U.S. Pat. No. 4,921,328 and the dissertation "Sol-Gel Processing of PZT Long Fiber", Walter Glaubitt, Dieter Sporn, and Rainer Jahn, Intelligent Materials and System (1995) pp. 47-54.
U.S. Pat. No. 4,921,328 covers an invention which concerns an inorganic polycrystalline ferroelectric fiber and a fiber-in-line optoelectronic modulator. It discloses a method for the production of a PLZT fiber by a sol-gel process using 2-methoxyethanol as a solvent, hydrated lead acetate, hydrated lanthanum acetate, zirconium propoxide, and titanium propoxide as metal sources, and nitric acid as a catalyst.
By this method of production disclosed in U.S. Pat. No. 4,921,328, the hydrated lead acetate and the hydrated lanthanum acetate as the raw materials are heated at 165.degree. C. for 15 minutes. This step is aimed at depriving the hydrates of the water of crystallization. It has been reported, however, that the removal of the water of crystallization results in producing a precipitate destined to constitute impurities J. Kang, T. Yoko, H. Kozuka, and S. Sakka, "Preparation of Pb-Based Complex Perovskite Coating Film by Sol-Gel Method", pp. 249-260, Proceedings of SPIE, Vol. 1758, Sol-Gel Optics II, San Diego, Calif., 1992!. The removal of the water of crystallization ought to be avoided to preclude the formation of the precipitate.
As a technique which shuns the removal of the water of crystallization, the method disclosed in the aforementioned dissertation by Glaubitt et al. may be cited. This method amounts to an improvement in the conventional method which resides in depriving the hydrate as the raw material of the water of crystallization while using 2-methoxyethanol as a solvent and converting what remains after the removal into a complex. It obviates the necessity for removing the water of crystallization by accomplishing necessary carboxylation by using propionic acid as a solvent The use of propionic acid, however, tends to induce generation of an unnecessary functional group. This functional group is coupled with such other groups as a carboxyl group and the product of the coupling is liable to gasify during the course of heating in the production of a fiber and compel the final product to assume a porous structure as noted from the SEM photograph of the final product annexed to the dissertation.
The optoelectronic modulation utilizing a fiber has been known from JP-A-60-170,828, for example. This patent publication discloses a PLZT optical shutter array element produced by the application of the technique of photolithography. This optical shutter array element is produced by chemically etching a flat plate of PLZT thereby incising grooves therein for the installation of an electrode, then depositing a metallic material on the side of the PLZT plate containing the grooves by means of vacuum evaporation, and again chemically etching the same side of the PLZT plate thereby imparting a prescribed pattern to the electrode.
JP-B-07-31,315 proposes an optical shutter array element which is produced by preparing a flat plate of PLZT having a thin metallic film, a precursor for an electrode, deposited thereon by vacuum evaporation and precision machining the PLZT plate with a diamond cutter thereby forming grooves of a prescribed depth as spaced with a prescribed pitch.
The optical shutter array element disclosed in JP-A60-170,828, however, is at a disadvantage in requiring an extremely complicated process of high-precision fabrication. It also has the problem that since the grooves are produced by chemical etching and, as a result, the grooves are not allowed to have a large depth and the light path is not allowed to have a large length, the shutter element inevitably requires a high voltage for its operation.
The optical shutter array element disclosed in JP-B07-31,315 has the problem that since the diamond cutter is used in fabricating the PLZT plate and, therefore, the pitch of the arrays cannot be decreased, the element allows compaction and high integration only with difficulty.